Variability in the Width of the Tropics and the Annular Modes

The dynamical interaction between the poleward edge of the Hadley cell and the mid-latitude eddy-driven jet is investigated using an idealised moist primitive equation aquaplanet model with grey radiation, a suite of full GCM's from the CMIP3 ensemble and ERA-Interim reanalysis. Recent studies have shown that in the real atmosphere and GCMs, the unforced variability of these two features are correlated in the Southern Hemisphere during austral summer. Here we show that the degree of interaction is determined by the time-mean seperation of the two features.

When the eddy-driven jet is well seperated from the poleward edge of the Hadley cell the correlation approaches zero. These observations are seen in the aquaplanet model, the CMIP3 ensemble and reanalysis data, with a similar decrease in correlation over the same range of jet seperations suggesting a robust dynamical effect.

A dynamical mechanism is provided to explain the observed correlation patterns. It is well known in the literature that movement of the eddy-driven jet are sustained by anomalous upper tropospheric eddy-momentum flux convergence. These changes are approximately balanced by the Coriolis torque on the meridional flow, as expected under quasigeostrophic scaling. The location of the anomalous meridional mass flux, relative to the climatological Hadley cell position, determines the extent to which the edge of the Hadley cell is influenced by changes in the eddy-driven jet, and hence whether the two features are correlated.